Union basics in C++ - Mini Project: Build & Apply

Union basics

📖 Scenario: Imagine you want to store either an integer or a floating-point number in the same place in memory, but never both at the same time. This helps save memory when you only need one value at a time.

🎯 Goal: You will create a union in C++ that can hold either an int or a float. Then, you will assign values and print them to see how unions work.

📋 What You'll Learn

Create a union named Number with two members: int i and float f.

Create a variable num of type Number.

Assign the integer value 42 to num.i.

Assign the floating-point value 3.14f to num.f.

Print the values of num.i and num.f after each assignment.

💡 Why This Matters

🌍 Real World

Unions are useful in low-level programming where memory is limited, such as embedded systems or device drivers.

💼 Career

Understanding unions helps in systems programming, working with hardware interfaces, and optimizing memory usage.

1

Create the union Number

Write a union named Number with two members: an int called i and a float called f.

C++

// Define the union Number with int i and float f
// Your code here

Hint

A union is like a struct but shares memory for all members. Use the keyword union followed by the name and curly braces.

2

Create a variable num of type Number

Declare a variable named num of type Number.

C++

union Number {
    int i;
    float f;
};

// Declare a variable num of type Number
// Your code here

Hint

To create a variable of a union type, write the union name followed by the variable name and a semicolon.

3

Assign integer and float values to num

Assign the integer value 42 to num.i. Then assign the floating-point value 3.14f to num.f.

C++

union Number {
    int i;
    float f;
};

Number num;

// Assign 42 to num.i
// Assign 3.14f to num.f
// Your code here

Hint

Use the dot operator to access union members and assign values like num.i = 42;.

4

Print the values of num.i and num.f

Print the values of num.i and num.f after the assignments using std::cout. Print num.i after assigning 42, then print num.f after assigning 3.14f.

C++

union Number {
    int i;
    float f;
};

Number num;

num.i = 42;
std::cout << "num.i = " << num.i << std::endl;

num.f = 3.14f;
std::cout << "num.f = " << num.f << std::endl;

// Your code here

Hint

Use std::cout to print text and values. Remember to include <iostream> and use std::endl for new lines.

Practice

1. What is the main characteristic of a union in C++?

easy

A. All members share the same memory location

B. Each member has its own separate memory

C. Unions can only store integers

D. Unions automatically initialize all members

Solution

1. Step 1: Understand union memory sharing

   A union allocates one shared memory block for all its members, so they overlap.

2. Step 2: Compare with other data structures

   Unlike structs, unions do not allocate separate memory for each member.

3. Final Answer:

   All members share the same memory location -> Option A

4. Quick Check:

   Union memory sharing = All members share the same memory location [OK]

Hint: Remember: union members overlap in memory [OK]

Common Mistakes:

• Thinking each member has separate memory
• Assuming unions initialize all members
• Believing unions only store one data type

2. Which of the following is the correct syntax to define a union named Data with an int and a float member?

easy

A. union Data = { int i; float f; };

B. struct Data { int i; float f; };

C. union Data ( int i; float f; );

D. union Data { int i; float f; };

Solution

1. Step 1: Recall union declaration syntax

   Unions are declared using the keyword union, followed by the name and braces with members inside.

2. Step 2: Check each option

   union Data { int i; float f; }; uses correct syntax with braces and semicolons. Options B, C, and D have syntax errors or wrong keywords.

3. Final Answer:

   union Data { int i; float f; }; -> Option D

4. Quick Check:

   Correct union syntax = union Data { int i; float f; }; [OK]

Hint: Use 'union Name { members; };' syntax [OK]

Common Mistakes:

• Using struct keyword instead of union
• Using parentheses instead of braces
• Assigning values in declaration

3. What will be the output of the following code?

union Number {
  int i;
  float f;
};

Number n;
n.i = 65;
std::cout << n.f << std::endl;

medium

A. A floating-point number representing the bits of 65

B. 65.0

C. 0

D. Compilation error

Solution

1. Step 1: Understand union memory sharing and type punning

   Assigning to n.i sets the bits of the union. Reading n.f interprets those bits as a float.

2. Step 2: Predict output

   The bits for integer 65 do not represent 65.0 as a float, so output is a float number corresponding to those bits, not 65.0.

3. Final Answer:

   A floating-point number representing the bits of 65 -> Option A

4. Quick Check:

   Union type punning = A floating-point number representing the bits of 65 [OK]

Hint: Union shares bits; int bits read as float show strange value [OK]

Common Mistakes:

• Expecting output 65.0
• Assuming union members hold independent values
• Thinking code causes compile error

4. Identify the error in this union usage:

union Data {
  int i;
  char c;
};

Data d;
d.i = 100;
d.c = 'A';
std::cout << d.i << std::endl;

medium

A. Output will be 100

B. Error: Cannot assign to union members separately

C. No error; output will be 65

D. Compilation error due to missing semicolon

Solution

1. Step 1: Understand union member assignment

   Assigning to d.i then to d.c overwrites the same memory because union members share memory.

2. Step 2: Determine final value of d.i

   After d.c = 'A', the first byte of d.i is set to ASCII 65, so printing d.i shows 65.

3. Final Answer:

   No error; output will be 65 -> Option C

4. Quick Check:

   Union last write wins = No error; output will be 65 [OK]

Hint: Last assigned union member overwrites memory [OK]

Common Mistakes:

• Thinking union members hold values independently
• Expecting output 100 after assigning 'A'
• Assuming syntax error

5. You want to store either an int, a float, or a char in the same memory space efficiently. Which union definition and usage correctly allows this, and how do you safely read the stored value?

hard

A. union Value { int i; float f; char c; }; Value v; v.i = 10; std::cout << v.c << std::endl;

B. union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl;

C. union Value { int i; float f; char c; }; Value v; v.c = 'Z'; std::cout << v.i << std::endl;

D. union Value { int i; float f; char c; }; Value v; std::cout << v.i << std::endl;

Solution

1. Step 1: Check union definition and assignment

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; defines a union with int, float, char and assigns a float value correctly.

2. Step 2: Verify safe reading of stored value

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; reads the same member v.f that was assigned, ensuring correct interpretation.

3. Step 3: Analyze other options

   Other options either assign to one member and read a different one, leading to unexpected values, or read uninitialized memory.

4. Final Answer:

   union Value { int i; float f; char c; }; Value v; v.f = 3.14f; std::cout << v.f << std::endl; -> Option B

5. Quick Check:

   Read the same union member you wrote [OK]

Hint: Always read the union member you last wrote [OK]

Common Mistakes:

• Reading a different member than assigned
• Using uninitialized union members
• Assuming union members hold independent values